This invention relates to heat-sealable blends of butene-1-ethylene copolymer and polypropylene. More particularly, the invention is directed to blends of butene-1-ethylene copolymer and about 2 to 9% by weight of polypropylene, which may be used to form film exhibiting improved heat sealability, processability and other properties.
A plastic film suitable for use in the fabricating of bags, shipping sacks and other items must possess a variety of good physical and mechanical properties. For example, it is advantageous if heat sealing of layers of the film may be conducted over a broad range of sealing temperatures while obtaining good seal strengths. It is also advantageous for the film to be strongly resistant to tearing and to exhibit good tensile strength. To be favorable economically, the film should be readily processable at high production speeds (line speeds) and an increase in film gauge should not be required to maintain satisfactory physical and mechanical properties.
As is known to those skilled in the art, heat seals between layers of butene-1 polymers have an unsatisfactory seal strength. Incorporating a small amount of low density polyethylene (LDPE) into polybutylene (PB), as disclosed in U.S. Pat. no. 3,634,551, improves heat seal strength somewhat. However, blends of a butene-1 polymer and LDPE form films which are not rapidly processable and have been found to require heat sealing over a relatively narrow range of temperatures. LDPE film itself, which enjoys wide commercial use, may be processed at higher production rates but has been found to exhibit low tear strength.
It has now been discovered that a blend of butene-1-ethylene copolymer and about 2 to 9% by weight of polypropylene may be used for the production of film with an excellent overall balance of properties and which overcomes many of the problems with the aforementioned prior art films.
U.S. Pat. No. 3,808,304 discloses heat sealable polypropylene/polybutylene blends, but obtains maximum seal strengths at polypropylene levels ranging from about 20 to 89% by weight. This high level of polypropylene addition may be expected to adversely affect other film properties, such as tear strength.
U.S. Pat. No. 4,075,290 discloses heat sealable blends of 80 to 99% by weight of polybutylene and 1 to 20% by weight of polypropylene. The disclosure, however, restricts the polybutylene component to butene-1 homopolymers and sets forth limited ranges of physical properties for both the polybutylene and polypropylene components.